In an electric power steering apparatus that drives a motor for steering assistance in accordance with a rotational operation of a steering member such as a steering wheel and transmits the rotational force of the motor to a steering mechanism thus to assist the steering, a steering torque applied to the steering member has to be detected for the drive control of the motor for steering assistance, and for such detection a torque detecting apparatus has been conventionally employed halfway of a steering shaft connecting the steering member and the steering mechanism.
In the torque detecting apparatus, the steering shaft, which is the object of the detection, is split into a first shaft and a second shaft coaxially connected through a torsion bar of a reduced diameter, so that when a steering torque is applied to the steering shaft by a rotational operation of the steering member, a relative angular displacement is created between the first and the second shaft along with distortion of the torsion bar caused by the effect of the steering torque, and the steering torque is detected based on the relative angular displacement.
The detection of the relative angular displacement between the first and the second shaft has been executed by various means, one of which is found in a torque detecting apparatus that includes a cylindrical magnet that integrally rotates with the first shaft and a magnetic yoke that integrally rotates with the second shaft, so as to detect the relative angular displacement utilizing a variation of a magnetic circuit between the cylindrical magnet and the magnetic yoke (For example, refer to Patent Document 1).
The magnetic yoke that integrally rotates with the second shaft includes a plurality of magnetic pole nails circumferentially aligned on a ring-shaped yoke element so as to axially extend in a same direction, such that two of such yoke elements opposing each other with the respective magnetic pole nails alternately aligned in a circumferential direction constitute a pair, and such pair is fixed on the second shaft. The cylindrical magnet that integrally rotates with the first shaft is constituted as a multi-pole magnet including the same number of magnetic poles circumferentially aligned thereon as that of the magnetic pole nails of the magnetic yoke, and is fixed on the first shaft at such a position that each of the magnetic pole nails of the paired magnetic yoke falls on a boundary between a N-pole and a S-pole.
On an outer side of the paired magnetic yoke, magnetic focusing rings that collect the magnetic flux generated on the magnetic yokes are fixedly disposed, so as to closely surround the outer side of the respective yoke element. The magnetic focusing rings respectively include a magnetic focusing projection, formed so as to project from a position circumferentially corresponding to each other and thus to oppose each other with a narrow airgap therebetween, and between the magnetic focusing projections, a magnetic sensor constituted of a hall element or the like is disposed.
Under such structure, when the relative angular displacement is created between the the first shaft and the second shaft, the circumferential positional relationship between the magnetic pole nails of the paired magnetic yoke and the magnetic poles of the cylindrical magnet changes in an opposite direction to each other, and a change of the magnetic flux in the respective magnetic yoke in accordance with the positional change increases or decreases the density of the magnetic flux leaking out into the airgap between the magnetic focusing projections, thereby changing an output of the magnetic sensor. Accordingly, picking up such change in output leads to detection of the torque applied to the first and the second shaft (steering torque).
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-149062